The present invention relates to a lighting device.
Lighting devices, such as a fluorescent lamp and a light emitting diode (LED), are generally designed to increase the photopic luminance for photopic vision under well-lit conditions. The human eye uses cone cells to sense brightness for photopic vision. Thus, a higher photopic luminance increases the sensitivity of the human eye to light.
However, for mesopic vision used in nighttime street and road scenarios, the human eye uses rod cells in addition to cone cells. The peak value of the spectral luminous efficiency for cone cells is 555 nm, whereas the peak value of the spectral luminous efficiency for rod cells is 507 nm. Thus, an increase in only the photopic luminance would not be effective. Such a shift in the spectral luminous efficiency is known as the Purkinje effect.
Japanese Laid-Open Patent Publication No. 2008-91232 describes an example of a lighting device that takes into consideration that the cone cells and the rod cells in the human retina both function under mesopic vision situations. The lighting device includes a plurality of light sources so that the cone cells and the rod cells are both able to function. At least one of the light sources emits light having a peak value in a wavelength band from between 450 to 550 nm, which includes 507 nm that is the peak wavelength of the spectral luminous efficiency for rod cells.
The lighting device takes into consideration the Purkinje effect and relatively increases the short wavelength components of visible light in the spectral distribution of the light source. This increases brightness sensitivity under mesopic vision situations. However, when plants, such as street trees, are illuminated with light adjusted to conform to mesopic vision situations, green leaves would appear bluish and unnatural. This would not be scenic.